This invention relates to a method of and apparatus for reducing the effect of noise in an electrical input signal which is obtained by scanning pictorial information, such as a television signal. In particular the invention is concerned with improvements in and modifications of the invention of U.S. Pat. No. 4,058,836.
Said earlier patent describes and claims a method of and apparatus for reducing the effect of noise in an electrical input signal which is obtained by scanning pictorial information on a field-by-field basis, to provide an output signal, in which a signal derived from a preceding-field output signal is subtracted from the input signal for the current field to provide a difference signal, low-amplitude portions of the difference signal are attenuated relative to high-amplitude portions thereof, and the thus-attenuated signal is added to the preceding-field output signal to provide an output signal for the current field.
As described in that patent, the attenuation is preferably achieved by a multiplier, one input of which is coupled to the subtractor output and the other input of which is coupled to the output of a non-linear transfer characteristic element which is also coupled to the subtractor output.
United States Patent Application Ser. No. 883,406 filed Mar. 6, 1978, now U.S. Pat. No. 4,194,219, (published as corresponding German Offenlegungsschrift 28 09 216) describes inter alia the use of a variable-gain element connected between the subtractor and the non-linear element.
The use of the variable gain element is of primary importance when the transfer characteristic of the non-linear element is such that below a predetermined value a constant maximum attenuation factor applies, and above this value the attenuation factor progressively reduces. The variable-gain element can then be adjusted manually so that the mean RMS noise level in the signal corresponds to the said predetermined value. The system is then optimally sensitive to motion.
This is a useful feature, but in practice the noise level in a signal can vary, particularly as between shots for example, and this would require continuous monitoring and adjustment to obtain the best results. This is impracticable.
We have also found that there is another problem associated with the variation in noise level across the grey scale. For example, gamma correction of thermally generated source noise, as in a camera, produces many times more noise amplitude at black than at white. On the other hand, gamma correction of signals produced by a telecine machine produces a film grain characteristic giving rise to noise which takes a peak value in the low level greys and is zero at black and white. If, as in these circumstances, the noise distribution over the gray scale is not uniform, then the setting of the variable-gain element can only be a compromise.